It has been known for some time that position-finding can be carried out on the basis of radio links, for example within the satellite-assisted GPS system (Global Positioning System). The European Galileo satellite navigation system, as well as positioning methods based on terrestrial radio sources, offer further applications. Position-finding methods such as these and appropriate appliances allow the user to determine his position by measurement of the distance to a specific number of wire-free signal sources such as satellites or base stations. For example, each GPS and Galileo satellite transmits unique digital sequences, which include a time identification and the satellite position. The signals are normally modulated with long spread sequences. The spread sequences for the individual satellites are virtually orthogonal with respect to one another, so that the signals can be distinguished from one another at the receiver. By way of example, the spread sequences for the various GPS and Galileo satellites are synchronized to one another by means of high-precision atomic clocks that are installed in the satellites.
The receiver evaluates the relative delays (delay time offsets) between different radio sources (GPS satellites, Galileo satellites or terrestrial transmitters). Together with the data about the position and the time reference of the various radio sources, the delay time offsets can be used in order to locate the receiver exactly. The receiver calculates the so-called pseudo-ranges, which represent the distance to each radio source. Navigation software can then calculate the user position on the basis of the pseudo-range to each radio source and the position of the radio sources (GPS satellites, Galileo satellites or terrestrial transmitters) by solving a set of non-linear equations.
The delay time offset between the received signals from the radio sources is frequently measured by determining a correlation maximum in a receiver architecture and assuming that this maximum corresponds to the direct line-of-sight (LOS) path. The problem is that the individual signals do not always reach the receiver along a direct line-of-sight transmission, but often are reflected or scattered by a large number of obstructions such as buildings or hills. These reflected or scattered signals propagate along a greater distance, however, and are therefore delayed. If no line-of-sight path exists, the measured correlation maximum does not correspond to the path delay of a line-of-sight path, and thus leads to a greater pseudo-range. Determination of the user position on the basis of these reflected and delayed signals thus leads to a position error.
Particularly in urban environments and environments within buildings as well as rural environments, it is known that there are reasons why the signals are reflected, diffracted and scattered, thus leading to multipath signal propagation, in which various versions of the signal arrive at different times at the receiver. Depending on the phase offset, the signals are subject to constructive or destructive interference, which leads to multipath fading. This effect can attenuate the line-of-sight signal and can lead to the receiver detecting a delayed version as a supposed maximum, instead of this. Furthermore, obstructions such as buildings in urban environments can shadow the line-of-sight signal to such an extent that the receiver detects a non-line-of-sight signal as the maximum. Multipath fading often leads to line-of-sight signals which are weaker than the non-line-of-sight signals. The severe attenuation, such as that which occurs in environments within buildings, can completely mask the line-of-sight signal with environmental noise, so that the receiver detects a non-line-of-sight signal.
This problem is generally not so important for wire-free communication, since the useful data contained in the received signals can often be recovered from the delayed signals. However, for position-finding systems, the essential information is contained in the time relationship between the signals from the various signal sources. Reflected and delayed signals propagate over a longer distance. The position-finding receiver will in consequence detect a greater pseudo-range than the line of sight range. An incorrect pseudo-range measurement leads to an incorrect position being found. Multipath propagation scenarios normally occur in built-up or mountainous environments, and thus lead to position-finding errors by the position-finding appliances.
Position-finding receivers that are currently in use, in particular GPS receivers with increased sensitivity, determine positions on the basis of all of the detectable satellite signals. A number of position-finding processes are carried out over a predetermined interval. If the so-called “Dilution of Precision” (DOP) for these position-finding processes is below a specific threshold value, the position that has been found is accepted, otherwise the measurements are rejected.
There are also other multipath avoidance techniques which are implemented in the antenna in order to constantly attenuate signals from specific directions. However, this restricts the antenna coverage and requires the antenna to be oriented in a specific manner, thus resulting in problems for hand-held receivers.
A further alternative is to exclude false pseudo-areas by comparison of the calculated user position with a-priori information about the approximate user position. When determining new, unknown positions, this can be done only with considerably reduced accuracy. Furthermore, this means that the position calculation must be carried out before a decision can be made about incorrect pseudo-areas, and thus results in significant additional costs.